Brain Size Linked to Longevity

Many mammals have proportionally large brains in comparison with their respective body sizes. Scientists have long hypothesized that those big brains, found in such animals as primates, cetaceans, and humans, must signify big intelligence. Now, researchers from the Centre for Ecological Research and Forestry Applications (CREAF), part of the Universitat Autònoma de Barcelona, have found a strong correlation between brain size and long life—and that longevity may be related to enhanced cognitive abilities.

Not a smart move, evolutionarily speaking

As animals evolved over the past few hundred million years, a rapid expansion of brain size occurred in some species—showing a change in the more linear ratio of body to brain found in lower invertebrates. Richard Emes, a researcher at the University of Nottingham, calls this tipping point an evolutionary oddity.

“From an evolutionary standpoint, it wasn’t a smart move,” Emes says. “Big brains are quite expensive and use a lot of energy. And you have to place an awful lot of emphasis on the social care of young while their big brains develop. It’s risky, so there must be some kind of selective pay off for brain mass.”

One proposed explanation for this tipping point is the cognitive buffer hypothesis (CBH). It’s possible that larger brains allow for learning and flexibility in behavior, and with that flexibility, animals can better handle environmental challenges like finding food or avoiding predators.

A comparison of mammals

Daniel Sol and César González-Lagos, two researchers at CREAF, and Simon Reader, a researcher at Utrecht University in the Netherlands, compared the brain sizes of nearly 500 different species of mammals, from bats to felines to cetaceans, to try to discover why some of these animals have bigger brains than others. In a study published in the May issue of the Journal of Evolutionary Biology, the team demonstrated that larger brain size was positively correlated with a longer lifespan. For example, the human brain is three times that of a chimpanzee despite having a similar body weight, and humans live twice as long.

“The correlation is definitely there,” says Sol. “But what we can’t say is if the larger brain is increasing the life span or the increased life span has something to do with the larger brain. Both possibilities are supported by the cognitive buffer hypothesis.”

It’s possible that a larger brain allows species to invent new behaviors to help deal with ecological challenges, thus increasing survival and lifespan. Or, a longer life may result in more crises like lack of food or exposure to predators which consequently push the development of new behaviors. Either way, Sol argues, the findings support the hypothesis.

Big brains and molecular complexity

But Seth Grant, a researcher at the Wellcome Trust Sanger Institute in the United Kingdom, says that one has to look beyond the brain before discussing plasticity and inventive new behaviors—understanding the molecular complexity of an animal’s synapses is important.

“There’s an assumption that the building blocks of bigger brains are neurons and the number of synapses. By adding them up, you have more,” he says. “It’s not that simplistic and logical.” He argues the synapses do a lot more than connect up—they are molecular computers, made up of thousands of proteins, processing information, regulating homeostasis, and making decisions. “Brain size is a vehicle to proliferate and multiply the number of these molecular computers available to an animal,” he says. “And that’s where it’s most useful.”

The pinnacle of evolution?

Emes argues that humans often think of themselves as the pinnacle of evolution—and we are mistaken. Simon Reader, a co-author on the Journal of Evolutionary Biology study, plans to take a closer look at the advantages of a smaller brain.

“It’s easy to focus on the advantages of large brains. As humans, we emphasize intelligence and cognition as the way forward,” Reader says. “But small-brained animals are doing quite well, and there may be some disadvantages of learning and cognition, too. We have to be able to explain both sides.”